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We knew this news was coming, perhaps. Now that it is here, it is no less shocking.

Ever since a historic coral bleaching event hit the treasured Great Barrier Reef in March - courtesy of a dramatic influx of warm water in the region - scientists have been trying to take a toll of the damage. And the latest report, from researchers with the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, seems to reaffirm some of the worst fears.

It's important to caution that not all of the evidence is in yet. The Great Barrier Reef is enormous and takes time to survey.

Still it appears that in the northern part of the Great Barrier Reef, large volumes of corals may have died. That's the part of the reef researchers say was, previously, the most "pristine" - in other words, the least damaged by pollution and other human influences.

"In the area [where] I am, I'm at Lizard Island, about 250 kilometres north of Cannes, around about 80 per cent and upwards of the corals have died," said Andrew Hoey, a senior research fellow with the Centre, during a break yesterday from the ongoing research.

In a press release from the ARC Centre, one of Hoey's colleagues, Greg Torda, said "millions" of corals in the northern sector of the reef have died.

Even though their studies are not complete, the researchers are already asserting that this is far worse than prior bleaching events that occurred in 1998 and 2002.

"The mortality is devastating really," said Hoey. "It's a lot higher than we had hoped."

Fortunately, the research has also shown that in the central and especially the southern parts of the reef, the damage has been considerably less severe. The Great Barrier Reef Marine Park Authority, for its part, estimates that 22 per cent of corals across the entirety of the reef have died, also affirming that the damage is worst in the north.

The first blow to the corals came in the form of the warm water itself. In an oceanic "weather" event that scientists have already strongly linked to climate change, water temperatures in the Coral Sea in March were record warm, about 1 degree Celsius above average. This afflicted the northern part of the Great Barrier Reef the most, however, as other sectors got a reprieve in the form of a cyclone that actually had a cooling effect on the water.

The stress caused by the warm ocean temperatures led to a severe bleaching event in the northern reef - the corals jettisoned the symbiotic algae that live within their cells and provide them with energy, turning white. In the northern part of the reef, air and underwater surveys in April showed that 93 per cent of all of the individual reefs comprising the Great Barrier Reef had seen at least some bleaching, and in the northern reef, 81 per cent of reefs showed bleaching that ranked as "severe".

The question, then, was simply how much coral would die - and how much would recover. Corals can recover from some bleaching, but in general, the more severe it is, the more risk that corals wouldn't make it through.

By May, diving and air surveys suggested as much as 50 per cent of the northern reef's corals may have died. And now, the researchers say, coral disease and coral eating snails have set in, causing additional devastation.

"A lot of the corals that appeared to survive the bleaching are now getting disease or predated upon by these snails," said Hoey from Lizard Island.

The ARC Centre has released a map showing where it has taken observations of corals, and including photos and videos to document the result.

More and more, researchers are saying this kind of impact amounts to climate change showing its hand. Research earlier this year suggested it was quite unlikely the warming event in the Coral Sea would have happened absent human influences on the atmosphere.

The development is directly relevant to international climate policy. The Earth right now is about 1 degree Celsius above its pre-industrial globally averaged temperature, according to scientists. And there is more warming in the pipeline, already baked in, due to our emissions.

The Paris climate change agreement, which enters into force early next month, lists two key thresholds for planetary temperatures - 2 degrees C above pre-industrial temperatures, which the agreement suggests must be avoided, and 1.5 degrees Celsius above them, which it suggests should be strived for as a limit to the extent of warming.

Yet if devastation of much of the Great Barrier Reef is the kind of event that can occur at 1 degrees C above pre-industrial levels (another may well be the destabilisation of the ice in West Antarctica), that suggests even these targets could mean major damage to global ecosystems from climate change.

Hoey said it would take 10 to 20 years for the reef's corals to recover from a blow like this one, but that would also require that they not get hit again by severe bleaching or other major disturbances in the meantime. The fear is that with ongoing warming, the insults will just keep coming.

"Coral reefs are always being subjected to some kind of disturbance, but they will bounce back," said Hoey. "But if they're being knocked over in too rapid succession, they just won't get back to where they were."